Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 31: Which of the following is a mitochondrial enzyme of the Urea Cycle?

A. Arginase
B. Carbamoyl Phosphate synthetase I (Correct Answer)
C. Arginosuccinate lyase
D. Arginosuccinate synthetase

Explanation: The Urea Cycle (Krebs-Henseleit cycle) is a metabolic pathway that occurs exclusively in the liver to detoxify ammonia. A key high-yield concept for NEET-PG is the **compartmentalization** of its enzymes between the mitochondria and the cytosol. ### 1. Why Carbamoyl Phosphate Synthetase I (CPS-I) is Correct The first two steps of the urea cycle occur within the **mitochondrial matrix**. * **CPS-I** is the rate-limiting enzyme that catalyzes the condensation of $NH_4^+$ and $HCO_3^-$ to form Carbamoyl Phosphate. * It requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * *Note:* The only other mitochondrial enzyme in this cycle is **Ornithine Transcarbamoylase (OTC)**. ### 2. Analysis of Incorrect Options * **Arginase (Option A):** This is the final enzyme of the cycle, located in the **cytosol**. It cleaves Arginine into Urea and Ornithine. * **Arginosuccinate Lyase (Option C):** Located in the **cytosol**, it cleaves arginosuccinate into arginine and fumarate (linking the urea cycle to the TCA cycle via the "aspartate-arginosuccinate shunt"). * **Arginosuccinate Synthetase (Option D):** Located in the **cytosol**, it condenses citrulline with aspartate. ### 3. High-Yield Clinical Pearls * **Mnemonic:** Remember "**M**other **O**nly" for Mitochondrial enzymes (**M**atrix): **M**-CPS-I and **O**-OTC. * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle. * **Most Common Deficiency:** **OTC deficiency** is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive). * **Hyperammonemia:** Defects in any of these enzymes lead to ammonia toxicity, presenting clinically with lethargy, vomiting, and cerebral edema.

Question 32: Which of the following statements is not true regarding tryptophan?

A. Involved in niacin synthesis
B. Involved in serotonin synthesis
C. Involved in melatonin synthesis
D. It is a non-essential amino acid. (Correct Answer)

Explanation: ### Explanation **Why Option D is the correct answer:** Tryptophan is an **essential amino acid**, not a non-essential one. Essential amino acids cannot be synthesized *de novo* by the human body and must be obtained through the diet. Tryptophan is one of the ten essential amino acids (remembered by the mnemonic **PVT TIM HALL**). It is also unique because it is both **glucogenic and ketogenic**, as its metabolism yields both pyruvate and acetoacetyl-CoA. **Why the other options are incorrect:** * **Option A (Niacin synthesis):** Tryptophan is a precursor for **Niacin (Vitamin B3)** via the Kynurenine pathway. Approximately 60 mg of tryptophan is required to synthesize 1 mg of niacin. This process requires Vitamin B6 (Pyridoxine) as a cofactor. * **Option B (Serotonin synthesis):** Tryptophan is hydroxylated by tryptophan hydroxylase (requiring Tetrahydrobiopterin/BH4) and then decarboxylated to form **Serotonin** (5-Hydroxytryptamine), a key neurotransmitter. * **Option C (Melatonin synthesis):** In the pineal gland, serotonin is further converted into **Melatonin**, the hormone responsible for regulating the circadian rhythm (sleep-wake cycle). **High-Yield Clinical Pearls for NEET-PG:** 1. **Hartnup Disease:** A genetic defect in the transport of neutral amino acids (like tryptophan) in the intestine and kidneys. It presents with **Pellagra-like symptoms** (Dermatitis, Diarrhea, Dementia) due to niacin deficiency. 2. **Carcinoid Syndrome:** Tumor cells divert up to 60% of tryptophan metabolism toward serotonin production. This leads to a deficiency in niacin synthesis, potentially causing secondary Pellagra. 3. **Diagnostic Marker:** 5-HIAA (5-Hydroxyindoleacetic acid) in urine is the breakdown product of serotonin and is used to diagnose Carcinoid tumors.

Question 33: Gamma-aminobutyric acid (GABA) is synthesized from which precursor molecule?

A. Fumarate
B. Glutamate (Correct Answer)
C. Histidine
D. Glycine

Explanation: ### Explanation **Correct Answer: B. Glutamate** **Mechanism:** Gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the central nervous system, is synthesized from **L-glutamate**. This conversion is catalyzed by the enzyme **Glutamic Acid Decarboxylase (GAD)**. This reaction is a decarboxylation process that requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. **Analysis of Incorrect Options:** * **A. Fumarate:** This is an intermediate of the TCA cycle and the urea cycle. While it can be used to regenerate aspartate, it is not a direct precursor to GABA. * **C. Histidine:** This amino acid is the precursor for **Histamine**, catalyzed by histidine decarboxylase (also B6 dependent). * **D. Glycine:** While glycine itself acts as an inhibitory neurotransmitter (primarily in the spinal cord), it is not a precursor for GABA. It is involved in the synthesis of heme, purines, and creatine. **Clinical Pearls for NEET-PG:** 1. **Vitamin B6 Deficiency:** Since GAD requires PLP (B6), a deficiency in Vitamin B6 leads to decreased GABA levels. This results in CNS over-excitation, manifesting as **convulsions** (commonly seen in infants). 2. **GABA Shunt:** This is a bypass of the TCA cycle where Glutamate → GABA → Succinic Semialdehyde → **Succinate**. This allows the brain to utilize GABA as an energy source during metabolic stress. 3. **Inhibitory Action:** GABA acts by increasing chloride conductance through GABA-A receptors, leading to hyperpolarization of the postsynaptic neuron.

Question 34: Which of the following is an essential amino acid?

A. Valine
B. Lysine
C. Threonine
D. All of the above (Correct Answer)

Explanation: **Explanation:** Amino acids are the building blocks of proteins, categorized into **Essential** and **Non-essential** based on the body's ability to synthesize them. Essential amino acids cannot be synthesized by the human body *de novo* at a rate sufficient to meet metabolic requirements and must, therefore, be obtained through the diet. **Why the correct answer is "All of the above":** All three options—**Valine, Lysine, and Threonine**—belong to the group of 10 essential amino acids. * **Valine** is a branched-chain amino acid (BCAA) vital for muscle metabolism and tissue repair. * **Lysine** is crucial for protein synthesis, hormone production, and calcium absorption. * **Threonine** is a principal component of structural proteins like collagen and elastin. **High-Yield Mnemonic for NEET-PG:** To remember the essential amino acids, use the mnemonic: **"PVT TIM HALL"** * **P:** Phenylalanine * **V:** **Valine** * **T:** **Threonine** * **T:** Tryptophan * **I:** Isoleucine * **M:** Methionine * **H:** Histidine (Semi-essential) * **A:** Arginine (Semi-essential) * **L:** Leucine * **L:** **Lysine** **Clinical Pearls for NEET-PG:** 1. **Semi-essential amino acids:** Arginine and Histidine are considered semi-essential because they are required in larger amounts during periods of rapid growth (infancy) or recovery from illness. 2. **Purely Ketogenic:** Leucine and Lysine are the only two amino acids that are purely ketogenic. 3. **Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PhIT**). 4. **Deficiency:** A diet lacking essential amino acids leads to conditions like **Kwashiorkor**, characterized by edema and "flaky paint" dermatosis.

Question 35: Which amino acid is regarded as the collector of amino groups?

A. Aspartate
B. Arginine
C. Cysteine
D. Glutamate (Correct Answer)

Explanation: **Explanation:** **Glutamate** is known as the "collector of amino groups" because of its central role in **transamination** and **deamination** reactions. During protein catabolism, most amino acids transfer their $\alpha$-amino group to $\alpha$-ketoglutarate (via aminotransferases), resulting in the formation of Glutamate. This process effectively funnels nitrogen from various amino acids into a single pool. Glutamate then undergoes **oxidative deamination** by the enzyme *Glutamate Dehydrogenase* in the mitochondria, releasing the amino group as free ammonia ($NH_3$) for the Urea Cycle. **Analysis of Incorrect Options:** * **Aspartate:** While it is an important nitrogen donor in the Urea Cycle (providing the second nitrogen atom to form Argininosuccinate), it is not the primary collector from all other amino acids. * **Arginine:** This is a semi-essential amino acid and an intermediate of the Urea Cycle. Its primary role is as a precursor for Nitric Oxide (NO) and Urea, not as a nitrogen collector. * **Cysteine:** This is a sulfur-containing amino acid involved in glutathione synthesis and disulfide bond formation; it does not play a central role in general amino group collection. **High-Yield Facts for NEET-PG:** * **Glutamine** is the major transport form of ammonia from peripheral tissues (like muscle and brain) to the liver. * **Alanine** is the primary transporter of amino groups from the muscle to the liver (Glucose-Alanine Cycle). * **Glutamate Dehydrogenase** is the only enzyme that can use either $NAD^+$ or $NADP^+$ as a coenzyme. * **$\alpha$-Ketoglutarate** is the universal acceptor of amino groups during transamination.

Question 36: Tyrosine becomes essential in which of the following conditions?

A. Wilson's disease
B. Alkaptonuria
C. Tyrosinosis
D. Phenylketonuria (Correct Answer)

Explanation: ### Explanation **1. Why Phenylketonuria (PKU) is the correct answer:** In normal metabolism, **Phenylalanine** (an essential amino acid) is converted into **Tyrosine** (a non-essential amino acid) by the enzyme **Phenylalanine Hydroxylase (PAH)**, using tetrahydrobiopterin ($BH_4$) as a cofactor. In Phenylketonuria, there is a deficiency of PAH or $BH_4$. This metabolic block prevents the synthesis of Tyrosine from Phenylalanine. Consequently, Tyrosine can no longer be produced endogenously and must be supplied through the diet to support protein synthesis and the production of neurotransmitters (dopamine, epinephrine) and melanin. Thus, Tyrosine becomes a **conditionally essential amino acid** in PKU patients. **2. Why the other options are incorrect:** * **Wilson’s Disease:** This is a disorder of copper metabolism (ATP7B mutation) and has no direct involvement in the phenylalanine-tyrosine metabolic pathway. * **Alkaptonuria:** This is caused by a deficiency of **Homogentisate oxidase**. While it is part of the tyrosine catabolic pathway, the block occurs *after* tyrosine is formed; therefore, tyrosine synthesis remains intact. * **Tyrosinosis (Tyrosinemia Type I):** This is caused by a deficiency of **Fumarylacetoacetate hydrolase**. Like Alkaptonuria, the block is in the downstream breakdown of tyrosine, not its initial synthesis. **3. Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** Characteristic of PKU due to phenylacetic acid in sweat and urine. * **Hypopigmentation:** PKU patients often have fair skin and blue eyes because tyrosine (the precursor to melanin) is deficient. * **Screening:** The Guthrie Test (bacterial inhibition assay) is the classic screening method for PKU. * **Dietary Management:** "Low Phenylalanine, High Tyrosine" diet is the mainstay of treatment.

Question 37: A 2-month-old infant presented with hepatic crisis, peripheral neuropathy, and Fanconi-like syndrome. Laboratory investigations showed elevated transaminases, low coagulation factors, and elevated succinylacetone in serum and urine. What is the most likely diagnosis?

A. Phenylketonuria
B. Homocystinuria
C. Tyrosinemia (Correct Answer)
D. Hawkinsinuria

Explanation: **Explanation:** The clinical presentation and laboratory findings point directly to **Tyrosinemia Type I** (Hepatorenal Tyrosinemia). **Why Tyrosinemia Type I is correct:** Tyrosinemia Type I is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**, the final step in the tyrosine degradation pathway. This deficiency leads to the accumulation of fumarylacetoacetate, which is diverted into the formation of **Succinylacetone**. * **Succinylacetone** is the pathognomonic marker for this condition; it is toxic to the liver (causing cirrhosis and hepatic crisis) and the kidneys (causing **Fanconi-like syndrome** with renal tubular acidosis). * It also inhibits heme synthesis, leading to porphyria-like neurological crises (**peripheral neuropathy**). **Why other options are incorrect:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. Presents with intellectual disability, "mousy" odor, and hypopigmentation, but not hepatic failure or succinylacetone elevation. * **Homocystinuria:** Caused by Cystathionine $\beta$-synthase deficiency. Characterized by ectopia lentis, marfanoid habitus, and thromboembolism. * **Hawkinsinuria:** A rare defect in the 4-hydroxyphenylpyruvate dioxygenase enzyme. It presents with failure to thrive and metabolic acidosis in infancy but lacks the severe hepatic crisis and succinylacetone marker. **NEET-PG High-Yield Pearls:** * **Diagnostic Marker:** Succinylacetone in urine/blood (Most specific). * **Treatment of Choice:** **Nitisinone (NTBC)**, which inhibits 4-hydroxyphenylpyruvate dioxygenase, preventing the formation of toxic metabolites. * **Long-term Risk:** Patients have a very high risk of developing **Hepatocellular Carcinoma (HCC)**. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine.

Question 38: Selenocysteine is structurally similar to which other amino acid?

A. Arginine
B. Alanine
C. Cysteine (Correct Answer)
D. Lysine

Explanation: **Explanation:** **Selenocysteine (Sec)**, often referred to as the **21st amino acid**, is an analogue of the proteinogenic amino acid **Cysteine**. 1. **Why Cysteine is Correct:** Structurally, Selenocysteine is identical to Cysteine, with one critical substitution: the sulfur atom in the thiol group (-SH) of Cysteine is replaced by a **selenium atom**, forming a **selenol group (-SeH)**. This substitution lowers the pKa of the side chain, making Selenocysteine a more efficient nucleophile and a potent catalyst in redox reactions. 2. **Why Other Options are Incorrect:** * **Arginine and Lysine:** These are basic, positively charged amino acids with long, complex side chains (guanidino and amino groups, respectively) that bear no structural resemblance to the simple three-carbon structure of Selenocysteine. * **Alanine:** While Alanine is the structural "backbone" of many amino acids, it lacks the reactive functional group (sulfur or selenium) found in Cysteine and Selenocysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Genetic Coding:** Unlike other non-standard amino acids, Selenocysteine is incorporated during translation via the **UGA stop codon**. This requires a specific mRNA secondary structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **tRNA:** It is synthesized while attached to its unique tRNA, **tRNA[Ser]Sec**. * **Key Selenoproteins:** Important human enzymes containing Selenocysteine include **Glutathione peroxidase** (antioxidant defense), **Thioredoxin reductase**, and **Deiodinase** (converts T4 to T3). * **Deficiency:** Low selenium levels can lead to **Keshan disease** (cardiomyopathy).

Question 39: Which amino acid is purely ketogenic?

A. Tyrosine
B. Leucine (Correct Answer)
C. Tryptophan
D. Isoleucine

Explanation: **Explanation:** Amino acids are classified based on the metabolic fate of their carbon skeletons into three categories: glucogenic, ketogenic, or both. **1. Why Leucine is Correct:** Leucine is one of only two **purely ketogenic** amino acids (the other being Lysine). Upon catabolism, Leucine is converted directly into Acetyl-CoA or Acetoacetate. These intermediates enter the ketogenic pathway to form ketone bodies and cannot be used for the net synthesis of glucose via gluconeogenesis. **2. Analysis of Incorrect Options:** * **Tyrosine (Option A):** This is **both glucogenic and ketogenic**. Its breakdown yields fumarate (glucogenic) and acetoacetate (ketogenic). * **Tryptophan (Option C):** This is **both glucogenic and ketogenic**. It is an aromatic amino acid that yields pyruvate and acetoacetyl-CoA. * **Isoleucine (Option D):** This is **both glucogenic and ketogenic**. It is a branched-chain amino acid (BCAA) that yields Succinyl-CoA and Acetyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Purely Ketogenic:** "The 2 L's" — **L**eucine and **L**ysine. * **Mnemonic for Both (Glucogenic & Ketogenic):** "PITTT" — **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, and **T**hreonine. * **Glucogenic:** All other 14 amino acids are primarily glucogenic. * **Clinical Correlation:** In conditions like Maple Syrup Urine Disease (MSUD), there is a defect in the metabolism of BCAAs (Leucine, Isoleucine, and Valine). Leucine is the most neurotoxic BCAA when levels are elevated.

Question 40: Which of the following conditions is caused by defective tyrosine metabolism?

A. Richner-Hanhart syndrome
B. Neonatal tyrosinemia
C. Alkaptonuria
D. All of the above (Correct Answer)

Explanation: **Explanation:** The metabolism of **Tyrosine** (a non-essential amino acid derived from Phenylalanine) involves a multi-step catabolic pathway. Defects in specific enzymes along this pathway lead to various clinical syndromes, all of which are categorized under disorders of tyrosine metabolism. * **Richner-Hanhart Syndrome (Tyrosinemia Type II):** Caused by a deficiency of the enzyme **Tyrosine Aminotransferase**. It is characterized clinically by the triad of palmoplantar hyperkeratosis, corneal erosions (pseudodendritic keratitis), and variable mental retardation. * **Neonatal Tyrosinemia:** This is usually a transient condition, especially in premature infants, caused by the delayed maturation of the enzyme **p-hydroxyphenylpyruvate dioxygenase (p-HPPD)**. It results in elevated levels of tyrosine and phenylalanine in the blood. * **Alkaptonuria:** A classic "inborn error of metabolism" caused by a deficiency of **Homogentisate Oxidase**. This leads to the accumulation of homogentisic acid, which causes darkening of urine upon standing, ochronosis (pigmentation of connective tissues), and arthritis. Since all three conditions arise from enzymatic blocks in the tyrosine degradative pathway, **Option D** is the correct answer. **NEET-PG High-Yield Pearls:** * **Tyrosinemia Type I (Hepatorenal type):** The most severe form, caused by a deficiency of **Fumarylacetoacetate hydrolase**. It leads to cabbage-like odor, liver failure, and renal rickets. * **Nitisinone (NTBC):** Used in the treatment of Tyrosinemia Type I to prevent the formation of toxic metabolites (succinylacetone). * **Diagnostic Test for Alkaptonuria:** Ferric chloride test (turns deep blue/green) and Benedict’s test (brown/black precipitate).

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

Practice Questions

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